Molecular oxygen reacts spontaneously with certain metal-peptide complexes in neutral solution to oxidize the peptides. Copper (II) and nickel (II) catalyze this activation of 02, but the copper (II) reaction is subject to photochemical inhibition. The oxidation mechanism is under investigation with the objective of determining how these spontaneous reactions are prevented or how they may be utilized for substate oxidations. Major aspects of the study are: (1) the oxygen reactivity as a function of amino acid sequence and number in polypeptides; (2) pH dependence; (3) determination of intermediate species formed in the oxidation; (4) analysis of the oxidation products as a function of polypeptides and substrates; (5) the role of the metal ion in the molecular oxygen activation and establishment of the subsequent reaction mechanism; (6) the nature of the photochemical inhibition.